(1) Field of the Invention
The present invention relates to an apparatus for detecting the intake pressure in an internal combustion engine. More particularly, the present invention relates to a technique of improving the precision of detection of the intake pressure which is used for controlling the engine such as the quantity of a fuel to be supplied into the engine.
(2) Related Art of the Invention
There is known an apparatus for controlling the supply of a fuel in an internal combustion engine, which comprises an intake presure (e.g. sucked negative pressure) detecting sensor disposed in a suction system, wherein the quantity of the fuel supplied to the engine is set and controlled according to the intake pressure detected by said sensor and the revolution number of the engine (see Japanese Unexamined Patent Publication No. 58-150040).
Since the intake pressure is caused to pulsate under influences of suction and exhaust of the cylinder, consequently, a timing for sampling the intake pressure significantly affects the detected value of the intake pressure.
The sampling timing of the intake pressure has been conventionally decided at predetermined crank angle positions, ignition timing or time intervals.
In the case where the intake pressure is sampled at a certain crank angle position, since the phase of the pulsation of the intake pressure to the crank angle position is greatly changed according to the engine load or the engine revolution number, a peak value of the pulsation or a central value of the pulsation is sampled according to the driving state, and therefore, the detected value of the intake pressure can not be reliable.
Further, in the case where the intake pressure is sampled at an output of the ignition signal, since the crank angle position at which the intake pressure is sampled is changed according to the change of the ignition timing, even if the engine load or revolution number is constant and the pulsation phase of the intake pressure is constant with respect to the crank angle position, the intake pressure to be sampled is changed. Accordingly, even if the fuel supply is controlled and set so that desired air-fuel ratio control characteristics can be obtained under the intake pressure sampled at a certain ignition timing, if the ignition timing is changed, the intake pressure sampled is greatly changed to degrade the air-fuel ratio control characteristics, and even if the intake pressure is constant, if the ignition timing is changed, a difference or gap is produced in the air-fuel ratio.
Furthermore, in the case where there is adopted a structure in which the intake pressure detected by a sensor is sampled at certain short time intervals (for example, 5 milliseconds), even if the time of one period of the intake pulsation at a reference revolution rate of, for example, 6000 r.p.m. is made in agreement with the time of the above-mentioned sampling period so that sampling is effected at a certain position of the pulsation (for example, the position traversing the central value of the pulse width at 6000 r.p.m., if the revolution rate shifts slightly from the reference revolution rate, for example, to 6001 r.p.m., the sampling position at the pulsation deviates gradually, creating an incremental increase of a measured intake pressure pulsation period. Over a long period, a pulsation period may become as long as several seconds to scores of seconds may be generated and an air-fuel ratio of the intake mixture as well as the exhaust state is periodically worsened. As is seen from the foregoing description, in the case where the air-fuel ratio changes at a long period, even if the weighted average of the sampled intake pressure is taken, it is difficult to obviate the above-mentioned pulsation of the air-fuel ratio.